Abstract

Cultures, predominantly of fibroblasts, were prepared from embryonic chick heart, newborn mouse skeletal muscle and adult guinea-pig subcutaneous granuloma. The tissues were dissociated with trypsin, and secondary cultures made on a glass surface of each tissue separately and of two mixtures (chick and mouse; chick and guinea-pig). The monolayering of these cultures, which it is believed is largely due to contact inhibition, was assessed quantitatively after 24 h by means of an overlap index. Homologous indices (overlapping of like cells) were obtained for all cultures, and heterologous indices (overlapping of unlike cells) for the mixed cultures. When cultured separately the different tissues differed considerably in their overlap indices. Increasing local density of population appears to make contiguous cells more ready to overlap each other. In the mixed cultures the homologous overlap indices had often distinctly different values from those of the single-tissue experiments. It is evident that the nature as well as the density of the immediately surrounding cell population affects the chance that an overlap will occur between any pair of cells in contact. The heterologous overlap index in the mixed cultures was in one case lower than either of the homologous indices; in the other case it was intermediate between the homologous indices, though not significantly different from the smaller one. It seems therefore unlikely that any simple generalization about overlap frequency, which will predict the results of untested combinations, will be possible. The mitotic index and nuclear area (probably representing degree of cell flattening) were also recorded, and in some cases, like the overlap index, showed variations according to the nature and density of the immediately surrounding cell population.